Simple Key Terms to Understand Relay Logic to PLC Logic Conversion

Control strategy: This is the sequence of steps that take place in a PLC program, which produces our desired output control.
Control task: It is our desired results of a control program.
Flowcharting: This is a method used to pictorially represent the operation of a process in a sequential manner.
Program coding: This is the process of translating a logic or relay diagram into the PLC ladder program form.
The implementation of a control program requires complex organizational and analytical skills, which change depending on the application. Because they are so varied, we cannot explain how to solve every specific control task.
Nevertheless, we can provide you with techniques and guidelines for completing this problem-solving process. In this handbook, we will introduce a strategy for implementing a control program, which includes program organization, system configuration, and I/O programming.
These strategies also apply to PLCs with the IEC 1131-3 programming standard. Additionally, we will present both simple and complex PLC programming examples. After you finish, you will be ready to learn how to document the PLC system—the last step in implementing the control program.

1. Define the Control Task

The control task should be defined by the user to begin the problem-solving process. This is basically defining what must be done. This information is the foundation of the control program. The control task should only be used by people who are familiar with the operation and processes of the machine. The success or failure of the control program directly affects the proper definition of the task.
There are many levels involved in the control task definition process. Each department must collaborate to identify the inputs required so everyone is clear about the project’s purpose and scope. If a project involves automation of a manufacturing facility, in which materials are to be retrieved from the warehouse, and then sent to the automated packaging area, both warehouse personnel and those working in packaging areas must work together with the engineering group during system definition.
If the project involves data reporting, management should be involved. The user should look at the steps used to control the task, whether it is done manually or via relay logic. This will help determine if there are any improvements that can be made. Relay logic can be implemented directly in a PLC. However, it is important to redesign the process to meet current project requirements and take advantage of the capabilities of programmable controllers.

2. Determine a Control Strategy

Once the control task is defined, it’s time to plan its solution. This involves the creation of a control strategy. This is the sequence of actions that must be taken within the program in order to achieve the desired output control. This is the process of developing an algorithm. It may seem strange or new to some, but algorithm is not. Everybody uses algorithms to complete certain tasks in their daily lives.
An algorithm is a process that allows a person to move from their home to school or work. The person leaves the house, gets in the car and starts the engine. He or she will reach the destination in the final of a finite number steps.
Implementing a PLC strategy for a control task closely matches the development of an algorithm. The control task must be implemented using a set of basic instructions. It should also be completed in a limited number of steps. If the task of creating an algorithm to solve the problem proves difficult, the user may need to go back to the control task definition in order to redefine the problem.
If we don’t know where we are and what Bullfrog County are, then we can’t explain how we get there. We need to determine if a specific method of transportation is necessary as part of the problem definition. We need to be aware of any time constraints. Without all the problem definition information, we cannot create a control strategy.
For program strategy definition, the fundamental rule is to think first and program later. You should consider other ways to solve the problem. Before you start programming the control function, it is important to refine the algorithm. This philosophy will reduce programming time reduce the debugging time, speed up start-up and put focus where it is needed – on design when designing and programming when programming.
The system designer must formulate a strategy, no matter if it is a new application of an existing process. The designer must analyze the sequence of events to optimize control by adding or removing steps. This will require knowledge of PLC-controlled field devices as well as input/output considerations.

3. Implementation Guidelines

Although a programmable controller can be a powerful device, it cannot do more than what is instructed to. The control program is the set of instructions and solution algorithms that the programmer has created for the controller. It gives it all the directions. The organization of the user will determine the success or failure of a PLC control programme.
There are many approaches to a problem. However, if an application is managed in a systematic way, there is less chance of making mistakes. The programmer will choose the method that is most appropriate for the program. However, it is important that the programmer follows certain guidelines.
Table1 contains programming guidelines for modernizations and new applications. Modernizations refer to new systems. New applications can be described as systems that are completely new. Modernizations can be used to upgrade existing control systems that were previously inoperative without a PLC (electromagnetic control or individual loop controllers).

New Applications Modernizations
  • Understand the desired function of the system.
  • Review possible control methods and optimize the process operation.
  • Flowchart the process operation.
  • Implement the flowchart by using logic diagrams or relay logic symbology.
  • Assign real I/O addresses and internal addresses to inputs and outputs.
  • Translate the logic implementation into PLC coding
  • Understand the actual process or machine function.
  • Review machine logic of operation and optimize when possible.

Table 1. Programming guidelines.

Understanding the machine’s operation or process is the first step to solve a control problem. The problem definition should be followed when developing a strategy for new applications. Revision of strategies for new applications and revisions to the control method for modernization projects will help you spot errors made during planning.
The programming stage is where you can see the differences between modernization and new projects. Modernization projects require that the user has a basic understanding of the machine’s operation and the task to be performed. A relay ladder diagram like the one in Figure 1 usually defines the sequence in the control program. This ladder diagram can almost be directly translated into PLC ladder charts.
New applications often begin with specifications that are given to the person responsible for designing and installing the control system. These specifications are translated into a written description which explains possible control strategies. To avoid confusion, the written explanation should be clear and concise. This explanation is then used by the designer to create the control program.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top